NEW YORK (Reuters) - The New York Stock Exchange website was inaccessible for 30 minutes on Monday, according to an Internet monitoring company, but the exchange said there was no interruption of service.

On Monday, the day hackers said they would attack the site, the NYSE website was unavailable from 5:30 p.m. EDT (2130 GMT) to 6 p.m. EDT (2200 GMT) and there was also an incident at 3:30 p.m. that lasted for about one minute, according to California-based Keynote Systems, which monitors websites.

A spokesperson for the NYSE said there was no interruption to web traffic and no sign of a hacker attack.

"We don't know if it was a hacker attack," Keynote spokesman Daniel Berkowitz said in an email. "We only see what end users see and so (we are) not sure if the cause was a DDOS (Distributed Denial of Service) attack."

A person familiar with the matter said on Tuesday that internal monitoring systems indicated the NYSE website had not experienced service issues and that the NYSE servers were not compromised.

The source said there may have been some slowness during the day originating from the external Internet service provider.

A video posted on YouTube, which claimed to be from the activist hacker group Anonymous, said the NYSE website would be "erased from the Internet" on Monday. The video said the move was in sympathy with the ongoing "Occupy Wall Street" protests in Lower Manhattan.

The threat was made against the NYSE website not the trading platform, which is used to process billions of share transactions each day. It was not possible to verify the origin of the threat.

So-called hactivists have used DDOS attacks, using supporters to crash websites by overwhelming the servers with traffic.

The Anonymous group launched DDOS attacks against Visa Inc <V.N> and MasterCard Inc recently, claiming the companies were hostile to WikiLeaks and its founder Julian Assange.